1. Field of the Invention
The present invention relates to a control unit for a torque converter for use in an automatic transmission for an automotive vehicle, and more particularly to a control unit for a torque converter with a lockup mechanism for locking and unlocking an impeller to and from a turbine.
2. Description of the Related Art
Torque converters are used in automatic transmissions for an automotive vehicles, and in many cases there is provided a lockup clutch for direct coupling an impeller and a turbine to the torque converter, whereby the power transmission efficiency is increased through engagement of the lockup clutch to aim at improving the fuel consumption further. In recent years, however, with a view to improving further the fuel consumption, there have been many cases where a control to partially engage the lockup clutch, that is, a slippage control of the lockup clutch is carried out. When the slippage control is carried out, the heating value from a friction material of the lockup clutch is increased and therefore the durability of a clutch friction material and a sealing member is easy to become an issue. Thus, the durability of such material and member needs to be secured.
To this end, conventionally there have been proposed various types of apparatuses and methods for protecting the lockup clutch from heat in association with the slippage of the lockup clutch. For example, JP-A-7-89365 discloses an apparatus for releasing a constant speed running control or a cruise control so as to protect a lockup clutch when the slippage of the lockup clutch becomes abnormally large during such a cruise control. In addition, JP-A-10-169771 discloses a control to reduce the engine speed when the temperature of a working oil in a torque converter becomes high abnormally.
In view of the durability of a lockup clutch, the durability of a clutch friction material is the most important, and the durability of the clutch friction itself is largely affected by the temperature thereof. Conventionally, a control to secure the durability of the clutch friction material is carried out based on the slippage of the clutch, the temperature of a torque converter fluid or the like. However, there has been a problem that such a control is carried out based on a temperature that is different from the temperature of an actual clutch friction material. To cope with this, conventionally, the safety factor of the durability of a clutch friction material has to be set larger so that the durability of the lockup clutch is not damaged even if the control thereof is carried out based on the slippage of the lockup clutch and the temperature of the torque converter. This, however, leads to the excessive quality of the lockup clutch and hence to the increase in production cost.
The present invention was made in view of the aforesaid problems and an object thereof is to provide a control unit for a torque converter with a lockup mechanism which allows the accurate evaluation of the durability of a lockup clutch friction material by accurately calculating the actual temperature of the lockup clutch friction material, so that a sufficient durability can be secured even with a small safety factor.
With a view to attaining the aforesaid object, a control unit for a torque converter with a lockup mechanism according to a first aspect of the invention is, as shown in FIG. 1, constructed such that the surface temperature of a clutch friction material of a lockup clutch is calculated by a surface temperature calculation unit B3, that the surface temperature of the clutch friction material so calculated and a predetermined first allowable temperature are compared with each other by a surface temperature comparison unit B4, and that when the surface temperature of the clutch friction material is judged to have reached or exceeded the first allowable temperature, a friction material temperature reduction operating unit B5 is then started to operate to reduce the surface temperature of the clutch friction material.
According to the construction described above, since the surface temperature of the clutch friction material is calculated accurately by the surface temperature calculation unit B3, the friction material temperature reduction operating unit B5 is started to operate to reduce the surface temperature of the clutch friction material when the surface temperature reaches or exceeds a temperature damaging the durability of the clutch friction material, in other words, the first allowable temperature, and therefore the durability of the lockup clutch can easily be secured while performing the slippage control of the lockup clutch. Note that the friction material temperature reduction operating unit is, for example, a unit fully locking up or unlocking the lockup clutch, and the control of such operations of the unit is carried out by a lockup operation control unit B6.
A control unit for a torque converter with a lockup mechanism according to a second aspect of the present invention is, as shown in FIG. 2, constructed such that the surface temperature of a clutch friction material of a lockup clutch is calculated by a surface temperature calculation unit B3, that the surface temperature of the clutch friction material so calculated and a predetermined second allowable temperature are compared with each other by a surface temperature comparison unit B11, and that a time when the surface temperature of the clutch friction material is judged to have reached or exceeded the second allowable temperature is accumulated and stored by a time accumulating unit B12, and that the lockup operation of the lockup clutch is regulated by a lockup operation regulation unit B13 since an accumulated time so accumulated and stored has reached or exceeded a predetermined allowable time. The regulation of the lockup operation is carried out by the lockup operation control unit B6.
In this apparatus, the second allowable temperature is a temperature which can be generated by a minor damage made to the friction material by the failure of the lockup operation regulation device. Since the clutch friction material is exposed to such a temperature as would cause such a damage for a long time, it cannot exhibit a predetermined performance, and therefore the lockup operation of the lockup clutch is regulated for securement of the safety. Note that it is desirable that a warning light is illuminated to indicate to the driver that the lockup clutch needs to be replaced so as to promote such a replacement.
It is preferred that the second allowable temperature is divided into a plurality of temperature ranges for plural setting, and that each of the plurality of second allowable temperatures is weighted such that the weight becomes heavier as the temperature increases, whereby a plurality of accumulated times are accumulated and stored so that whether or not the lockup operation of the lockup clutch should be carried out is judged based on the accumulated time. This enables the more accurate grasp of the degree of damage to the lockup clutch, thereby making it possible to make a more suitable judgment on the needed replacement.
In the apparatus described above, as shown in FIGS. 1 and 2, preferably the surface temperature calculation unit B3 performs operations for the surface temperature of the clutch friction material based on the temperature of the working oil in the torque converter detected for example by a fluid temperature sensor B1 and a heating value in the lockup clutch calculated by a heating value calculation unit B2, thereby making it possible to obtain substantially accurately the surface temperature of the clutch friction material.